1. Technical Field
This invention is generally related to novel nanowire catalysts and, more specifically, to nanowires useful as heterogeneous catalysts in a variety of catalytic reactions, such as the oxidative coupling of methane to C2 hydrocarbons.
2. Description of the Related Art
Catalysis is the process in which the rate of a chemical reaction is either increased or decreased by means of a catalyst. Positive catalysts increase the speed of a chemical reaction, while negative catalysts slow it down. Substances that increase the activity of a catalyst are referred to as promoters or activators, and substances that deactivate a catalyst are referred to as catalytic poisons or deactivators. Unlike other reagents, a catalyst is not consumed by the chemical reaction, but instead participates in multiple chemical transformations. In the case of positive catalysts, the catalytic reaction generally has a lower rate-limiting free energy change to the transition state than the corresponding uncatalyzed reaction, resulting in an increased reaction rate at the same temperature. Thus, at a given temperature, a positive catalyst tends to increase the yield of desired product while decreasing the yield of undesired side products. Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated or destroyed by secondary processes, resulting in loss of catalytic activity.
Catalysts are generally characterized as either heterogeneous or homogeneous. Heterogeneous catalysts exist in a different phase than the reactants (e.g. a solid metal catalyst and gas phase reactants), and the catalytic reaction generally occurs on the surface of the heterogeneous catalyst. Thus, for the catalytic reaction to occur, the reactants must diffuse to and/or adsorb onto the catalyst surface. This transport and adsorption of reactants is often the rate limiting step in a heterogeneous catalysis reaction. Heterogeneous catalysts are also generally easily separable from the reaction mixture by common techniques such as filtration or distillation.
In contrast to a heterogeneous catalyst, a homogenous catalyst exists in the same phase as the reactants (e.g., a soluble organometallic catalyst and solvent-dissolved reactants). Accordingly, reactions catalyzed by a homogeneous catalyst are controlled by different kinetics than a heterogeneously catalyzed reaction. In addition, homogeneous catalysts can be difficult to separate from the reaction mixture.
While catalysis is involved in any number of technologies, one particular area of importance is the petrochemical industry. At the foundation of the modern petrochemical industry is the energy-intensive endothermic steam cracking of crude oil. Cracking is used to produce nearly all the fundamental chemical intermediates in use today. The amount of oil used for cracking and the volume of green house gases (GHG) emitted in the process are quite large: cracking consumes nearly 10% of the total oil extracted globally and produces 200M metric tons of CO2 equivalent every year (Ren, T, Patel, M. Res. Conserv. Recycl. 53:513, 2009). There remains a significant need in this field for new technology directed to the conversion of unreactive petrochemical feedstocks (e.g. paraffins, methane, ethane, etc.) into reactive chemical intermediates (e.g. olefins), particularly with regard to highly selective heterogeneous catalysts for the direct oxidation of hydrocarbons.
While there are multistep paths to convert methane to certain specific chemicals using first; high temperature steam reforming to syngas (a mixture of H2 and CO), followed by stochiometry adjustment and conversion to either methanol or, via the Fischer-Tropsch (F-T) synthesis, to liquid hydrocarbon fuels such as diesel or gasoline, this does not allow for the formation of certain high value chemical intermediates. This multi-step indirect method also requires a large capital investment in facilities and is expensive to operate, in part due to the energy intensive endothermic reforming step. (For instance, in methane reforming, nearly 40% of methane is consumed as fuel for the reaction.) It is also inefficient in that a substantial part of the carbon fed into the process ends up as the GHG CO2, both directly from the reaction and indirectly by burning fossil fuels to heat the reaction. Thus, to better exploit the natural gas resource, direct methods that are more efficient, economical and environmentally responsible are required.
One of the reactions for direct natural gas activation and its conversion into a useful high value chemical, is the oxidative coupling of methane (“OCM”) to ethylene: 2CH4+O2→C2H4+2H2O. See, e.g., Zhang, Q., Journal of Natural Gas Chem., 12:81, 2003; Olah, G. “Hydrocarbon Chemistry”, Ed. 2, John Wiley & Sons (2003). This reaction is exothermic (ΔH=−67 kcals/mole) and has typically been shown to occur at very high temperatures (>700° C.). Although the detailed reaction mechanism is not fully characterized, experimental evidence suggests that free radical chemistry is involved. (Lunsford, J. Chem. Soc., Chem. Comm., 1991; H. Lunsford, Angew. Chem., Int. Ed. Engl., 34:970, 1995). In the reaction, methane (CH4) is activated on the catalyst surface, forming methyl radicals which then couple in the gas phase to form ethane (C2H6), followed by dehydrogenation to ethylene (C2H4). Several catalysts have shown activity for OCM, including various forms of iron oxide, V2O5, MoO3, Co3O4, Pt—Rh, Li/ZrO2, Ag—Au, Au/Co3O4, Co/Mn, CeO2, MgO, La2O3, Mn3O4, Na2WO4, MnO, ZnO, and combinations thereof, on various supports. A number of doping elements have also proven to be useful in combination with the above catalysts.
Since the OCM reaction was first reported over thirty years ago, it has been the target of intense scientific and commercial interest, but the fundamental limitations of the conventional approach to C—H bond activation appear to limit the yield of this attractive reaction. Specifically, numerous publications from industrial and academic labs have consistently demonstrated characteristic performance of high selectivity at low conversion of methane, or low selectivity at high conversion (J. A. Labinger, Cat. Lett., 1:371, 1988). Limited by this conversion/selectivity threshold, no OCM catalyst has been able to exceed 20-25% combined C2 yield (i.e. ethane and ethylene), and more importantly, all such reported yields operate at extremely high temperatures (>800 C).
In this regard, it is believed that the low yield of desired products (i.e. C2H4 and C2H6) is caused by the unique homogeneous/heterogeneous nature of the reaction. Specifically, due to the high reaction temperature, a majority of methyl radicals escape the catalyst surface and enter the gas phase. There, in the presence of oxygen and hydrogen, multiple side reactions are known to take place (J. A. Labinger, Cat. Lett., 1:371, 1988). The non-selective over-oxidation of hydrocarbons to CO and CO2 (e.g., complete oxidation) is the principal competing fast side reaction. Other undesirable products (e.g. methanol, formaldehyde) have also been observed and rapidly react to form CO and CO2.
In order to result in a commercially viable OCM process, a catalyst optimized for the activation of the C—H bond of methane at lower temperatures (e.g. 500-700° C.), higher activities and higher pressures are required. While the above discussion has focused on the OCM reaction, numerous other catalytic reactions (as discussed in greater detail below) would significantly benefit from catalytic optimization. Accordingly, there remains a need in the art for improved catalysts and, more specifically, a need for novel approaches to catalyst design for improving the yield of, for example, the OCM reaction and other catalyzed reactions. The present invention fulfills these needs and provides further related advantages.